I assume your bivy sack will be protected from the rain by a tarp or tent; otherwise life will suck if it rains.
Either insulation will work in a bivy sack, but my concern is that you're using a WPB material. In my experience, WP fabrics don't breathe sufficiently to put over a sleeping bag (UNLESS, you're wearing a VBL sack or clothing inside the sleeping bag to prevent moisture from escaping from your body into the bag's insulation). Even my highly breathable non-WP Pertex Quantum bivy collects moisture on the inside.
WPBs require heat to drive the moisture through them. In a jacket, your body (just mm away from the fabric) does the job. With a sleeping bag, the great distance between your body and the WPB fabric creates a temperature gradient wherein the fabric temperature on the bag side of the bivy is allowed to drop below the dew point (the temperature at which the relative humidity reaches 100%). When this occurs the water vapor condenses into liquid water (or ice) on the inside of the bivy sack rather than passing through the "WPB" fabric.
In this case a synthetic bag would best retain insulative powers as the moisture continues to build inside the insulation during the night and continue doing this into your second night.
Rather than creating this scenario, how about a nice, light down bag with a light, breathable non-WP bivy sack and a tarp or well vented tent for rain protection?